1. Field of the Invention
Embodiments of the invention described herein pertain to the field of submersible pump assemblies. More particularly, but not by way of limitation, one or more embodiments of the invention enable an electric submersible pump inverted shroud assembly.
2. Description of the Related Art
Submersible pump assemblies are used to artificially lift fluid to the surface in deep wells such as oil or water wells. A typical vertical electric submersible pump (ESP) assembly consists of, from bottom to top, an electrical motor, seal section, pump intake and centrifugal pump, which are all connected together with shafts. The electrical motor supplies torque to the shafts, which provides power to the centrifugal pump. The electrical motor is generally connected to a power source located at the surface of the well using a motor lead cable. The entire assembly is placed into the well inside a casing. The casing separates the submersible pump assembly from the well formation. Perforations in the casing allow well fluid to enter the casing. These perforations are generally below the motor and are advantageous for cooling the motor when the pump is in operation, since fluid is drawn passed the outside of the motor as it makes it way from the perforations up to the pump intake.
One challenge to economic and efficient ESP operation is pumping gas laden fluid. When pumping gas laden fluid, the gas may separate from the other fluid due to the pressure differential created when the pump is in operation. If there is a sufficiently high gas volume fraction, typically about 10% or more, the pump may experience a decrease in efficiency and decrease in capacity or head (slipping). If gas continues to accumulate on the suction side of the impeller it may entirely block the passage of other fluid through the centrifugal pump. When this occurs the pump is said to be “gas locked” since proper operation of the pump is impeded by the accumulation of gas. As a result, careful attention to gas management in submersible pump systems is needed in order to improve the production of gas laden fluid from subsurface formations.
Currently in wells with gas laden fluid, and particularly in low volume, high gas wells (typically 200-500 bpd and 700-1000 MCF/d), a conventional inverted shroud is sometimes employed. In such instances, a shroud is placed around the ESP motor, enclosing the motor within the shroud, and including tubing that extends upwards towards the pump base. The bottom of the shroud around the motor is closed, creating a barrier to well fluid. The top of the shroud is open, typically attached to the pump base just above the intake. During operation, the well fluid enters perforations in the well casing located below the motor. The well fluid travels upwards in between the shroud and well casing. At the top of the shroud near the pump base, the fluid makes a 180° turn, and travels down the inside of the shroud, between the shroud and the pump assembly, and into the pump intake. From the pump intake, the fluid enters the pump and is carried through production tubing to the surface. As the fluid makes its turn at the top of the shroud, a portion of the gas breaks out of the laden fluid prior to entry into the pump, and naturally rises to the surface. The liquid travels downwards towards the intake.
A drawback to the use of conventional inverted shrouds is that, since the motor is inside the shroud, well fluid bypasses the motor in its path through the pump assembly. Without cooling well fluid flowing around the motor, the motor risks overheating or failure due to the lack of cool, fresh flowing fluid passing by. One approach to cooling the motor in ESP assemblies making use of inverse shrouds is a recirculation pump. The problem with recirculation pumps is that they require a thin-walled and fragile recirculation tube. This recirculation tube is easily pinched or broken. The fragile nature of the recirculation tube requires a very careful and slow installation process. If the recirculation pump fails, the motor may overheat, leading to failure. In addition, recirculation pumps are expensive since they require an additional pump be added into the ESP assembly.
It would be an advantage for submersible pump assemblies making use of inverted shrouds to be better suited to keeping the motor cool. Therefore, there is a need for an improved inverted shroud assembly.